This application is based on Application No. 2000-304935 filed in Japan on Oct. 4, 2000, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an optical module, specifically, to an optical module in which a method of holding a pyroelectric optical crystal mounted to, e.g., an optical semiconductor laser module is improved.
2. Description of the Related Art
Descriptions will be given with reference to FIG. 3 on a conventional method of fixing an optical crystal in an optical module and the operation thereof.
FIG. 3 shows an optical crystal holding portion in the case where a pyroelectric optical crystal is used in an optical module.
In FIG. 3, reference symbol 1 denotes a pyroelectric LiNbO3 crystal (lithium niobate, hereinafter referred to as LN crystal), reference symbol 2 denotes a substrate, 3, an adhesive, and 4, polarization charges generated on the surface of the crystal due to the pyroelectricity of the LN crystal 1. Denoted by reference symbol 5 is a laser diode, 6, a carrier for fixing the laser diode 5 to the top face of the substrate 2, and 7, signal light emitted from the laser diode 5.
The LN crystal 1 is a birefringent crystal. It works on the signal light emitted from the laser diode 5 to change the polarization of the light 7, and the amount of change in polarization is dependent upon the wavelength. Therefore the LN crystal 1 is used as a wavelength discriminating element. The adhesive 3 is used to fix the LN crystal 1 to the substrate 2 in the optical module.
As described above, conventional optical modules directly apply an adhesive to a substrate in the module to fix optical components to be mounted in the module. Otherwise, the conventional optical modules use a holder to fix the components by tightening a screw or by soldering.
However, fixing a pyroelectric crystal with the use of an adhesive has a problem, for it is incapable of keeping the optical characteristic of the crystal stable for a long period of time as detailed below.
The LN crystal 1, the substrate 2 and the adhesive 3 have different linear expansion coefficients. In a long term use, minute changes in temperature in the LN crystal 1 lead to accumulation of stress due to the difference in linear expansion coefficient among the three in the LN crystal 1. As shown in FIG. 3, the stress accumulated in the LN crystal 1 causes the pyroelectricity of the LN crystal 1 to generate polarization charges 4 on the surface of the crystal, and then the generated polarization charges 4 create an electric field. It has been found that the created electric field affects the LN crystal 1, with the result that its optical characteristic, e.g., birefringent characteristic is changed, and that the change causes variation with age in the aforementioned wavelength discriminating characteristic.
As described above, the conventional optical modules have a problem of being incapable of providing a long term stability in the operation characteristic as an optical module due to the change with age in characteristic of the optical crystal.
Furthermore, it is impossible to predict this optical characteristic change under the influence of the electric field because the change in optical characteristic is caused by application of slight stress to the bonded portion and by the remaining stress thereof. The change with age in optical characteristic thus cannot be compensated by other mechanisms, adding another problem to the conventional optical modules.
The present invention has been made to solve these problems, and an object of the present invention is therefore to provide an optical module capable of keeping the optical characteristic of a pyroelectric crystal stable for a long period of time even when the crystal is fixed by an adhesive.
According to the present invention, an optical module comprising a pyroelectric optical crystal transmitting signal light, and a substrate to which the optical crystal is fixed, is characterized in that a conductive film is formed on at least one face of the optical crystal.
Also, the optical module is characterized in that a conductive film having a transparent region with respect to the wavelength of the signal light is formed on at least one face of the optical crystal through which the signal light is transmitted.
Furthermore, the optical module is characterized in that the film formed on the signal light transmissive face of the optical crystal is formed from a material that makes the film a non-reflective coating with respect to the signal light.